1. Field of the Invention
The present invention relates to a method for evaluating the chemical stability of a polychloroprene latex. More specifically, it relates to a method for evaluating the chemical stability of a polychloroprene latex used in dip-formed articles such as gloves, boots, and balloons.
2. Description of the Related Art
Polychloroprene latexes have been used as the materials for producing dip-formed articles such as medical gloves, laboratory gloves, catheters, rubber boots, rubber threads, and balloons. These dip-formed articles have been produced mainly by a so-called coagulant solution method, using a three-dimensional former of ceramic or metal. Specifically, a former coated with a coagulant solution, which coagulates polychloroprene latexes, is immersed into and withdrawn from a polychloroprene latex composition blended with sulfur, a vulcanization accelerator and others and then dried, forming a film on the surface. The film is then stripped from the former, to give an article in a particular shape.
In a process for producing dip-formed articles with thin film thickness, such as surgery gloves, there are occasionally problems of coagulation of polychloroprene latex composition during its preparation and cracking and pinhole generation in finished articles. For reduction of such troubles, it is effective to control the stability of the polychloroprene latex to chemicals (chemical stability). For example when the chemical stability of a polychloroprene latex is too low, there may be coagulation when a polychloroprene latex composition is prepared, as it is blended with sulfur or a vulcanization accelerator. Alternatively when the chemical stability is too high, it is not possible to form a uniform film on the former surface, thus leading to easier cracking and pinhole generation thereon.
The chemical stability of a rubber latex is generally determined by mixing the latex with an aqueous inorganic salt solution and measuring the concentration and the dry mass of the coagulum generated (see Patent Documents 1 to 4). For example in the method for producing a polymer latex described in Patent Document 1, determined is the dry mass of the aggregate generated when 5 ml of aqueous 2.5 mass % calcium chloride solution is added to 10 g of a polymer latex prepared by emulsion polymerization of styrene, ethyl acrylate, butyl acrylate, or methyl methacrylate. The ratio of the aggregate to the polymer (mass %) is then determined and used for evaluation of the chemical stability.
Alternatively, Patent Document 2 describes a method of evaluating the chemical stability by determining the amount of the coagulum generated when 5 g of aqueous 0.1-N calcium chloride solution is added dropwise to 100 g of a copolymer latex having a solid matter concentration of 48 mass %, as the mixture is agitated. Yet alternatively, Patent Document 3 describes a method for evaluating the chemical stability by determining the amount of the coagulum generated when aqueous 7 mass % potassium hydroxide solution is added dropwise at a rate of 10% to 100 g of a copolymer latex (solid matter content: 45 mass %) previously filtered.
On the other hand in the method for evaluating the chemical stability described in Patent Document 4, a drop of a latex composition for wet master batch (about 0.2 cm3) was dropped on aqueous sodium chloride solutions different in concentration at an interval of 0.1 mass % and the NaCl solution at the highest concentration in which the latex composition does not solidify is determined.